In general, austenitic steel has a variety of applications, due to the properties thereof, such as work hardenability and non-magnetism. Conventionally used carbon steel, having ferrite or martensite as a major structure thereof, has limitations in terms of the properties thereof, and thus, due to being a material able to overcome the disadvantages of carbon steel, the applications of austenitic steel are increasing.
Fields of application of austenitic steel, include non-magnetic structural materials for a general electrical devices and superconductive devices, such as linear induction motor rails and nuclear fusion reactors, and equipment for mining, transportation, and storage in the oil and gas exploration industries that require steel having ductility, wear resistance, and hydrogen embrittlement resistance of expandable pipe steel, slurry pipe steel, and sour gas resistant steel. In these fields, the demand for austenitic steel is continually increasing.
AISI304 (18Cr-8Ni) is a typical conventional austenitic stainless steel. However, this steel has low yield strength, and thus, is difficult to use as a structural material. In addition, since this steel includes Cr and Ni, relatively expensive alloying elements, in large quantities, it is economically unfeasible to use in large amounts, and thus, there are limits to the applications thereof.
The content of manganese and the content of carbon may be increased to form austenite microstructures in austenitic steel, and in particular, in addition to carbon, an amount of Cr may be significantly increased to maintain a high degree of strength in the austenitic steel. In this case, carbides having a network form may be formed along austenite grain boundaries of the austenitic steel at high temperature, and thus, the physical properties of steel, particularly ductility, may be significantly reduced. In addition, carbides may be heavily formed in a weld heat-affected zone, heated to a high temperature during welding and rapidly cooled thereafter, as in the case of a base metal, thereby significantly decreasing toughness in such a weld heat-affected zone.
In order to suppress the precipitation of carbides having a network form, a method for preparing high manganese steel featuring a solid solution treatment at a high temperature or performing hot working and quenching to room temperature has been suggested. However, in the case in which steel is relatively thick, there are disadvantages in that the effect on suppressing the generation of carbides by quenching may be insufficient, and also, it may be difficult to prevent the precipitation of carbides in a welding heat-affected zone affected by a new heat history.